Fig. 757.
To prevent the ends of the set-screw or screws of the driver from damaging the surface of finished work, the form of driver shown in [Fig. 757] has been patented in England. It consists of a disc arched to receive a lever c, which is pivoted in the disc at d. A set-screw provided in the disc binds one end of the lever to the work, and as the pressure to drive the work is applied at the other end of the same lever, it serves to assist (to some extent) the set-screw in binding the lever to the work. The work is held between a V in the disc and one on the lever, the object being to provide a large area of contact, and thus prevent the damage to finished work which screw ends are apt to cause.
Fig. 758.
The same end may be obtained for ordinary drivers by using a copper or brass ring, such as shown in [Fig. 758], which may be opened or closed, within certain limits, to suit the diameter of the work, being placed on the end of the work, and within the dog, to receive the pressure of the set-screws.
One such ring will serve for several diameters of work, springing open when forced, under hand pressure, upon the work, or closing upon the work as the pressure of the dog set-screw is received. It is obvious that the split of the ring should be placed diametrally opposite to the dog set-screw.
